The genomic positions for the early and late mRNAs of BK virus, a human papovavirus, were determined. The 5' ends of the two early mRNAs produced in lytically infected human embryonic kidney cells, and in BKV transformed human cells, are located at approximately 0.66 map unit; their 3' ends are at 0.16 map unit. Intervening genomic sequences deleted from one early species lie between 0.585 and 0.53 map units and from the other, between 0.535 and 0.53 map units. The genomic body sequences for the 16S species of late BKV RNA are positioned between 0.93 and 0.16 map units. Sequences for the 19S species of late RNA are between 0.765 and 0.16 map units. A leader sequence transcribed from 0.70 to 0.76 units of the viral genome was detected. The structure of late BKV RNAs synthesized in African green monkey cells appeared to be identical to that from human embryonic kidney cells. A comparison of these data with those for SV40 shows both viruses to be closely related.